Study explains Bay Area's higher breast cancer

Study explains Bay
Area's higher breast cancer rate

BY TIM STEPHENS

Ever since a 1994 report identified
white women in the Bay Area as having the world's highest incidence
of breast cancer, many have worried that some unknown hazard in the
local environment might be increasing their susceptibility to this
disease. Now, a study by Stanford epidemiologists should help
dispel that concern.

The higher incidence of breast
cancer here is entirely explainable in terms of known risk factors
prevalent among women who live in the Bay Area, the new study
shows.

Childbearing patterns ­
specifically, the number of children a woman has had and the age at
which she bore her first child ­ account for most of the
difference in breast cancer rates between the Bay Area and the rest
of the country, the researchers report in the July 2 Journal of the
National Cancer Institute.

"Women in the Bay Area tend to have
fewer children, and to have them at later ages, than women in the
rest of the United States, and those are well-established risk
factors," explained Dr. Anthony Robbins, a physician pursuing a PhD
in epidemiology at Stanford.

Robbins performed the study with
Jennifer Kelsey, chief of epidemiology and professor of health
research and policy, and statistician Sonia Brescianini.

The annual breast cancer incidence
in the Bay Area is 114.6 new cases for every 100,000 women who have
never before been diagnosed with the disease, according to the
National Cancer Institute's 1988-1992 estimates. This figure, which
includes women of all ethnic backgrounds, is approximately 25
percent greater than that of women in New Mexico, who have the
nation's lowest incidence.

Within the Bay Area (defined in the
study as the counties of San Francisco, Alameda, Contra Costa,
Marin and San Mateo) there are also differences between breast
cancer rates for women of different ethnic backgrounds. For
example, black women in the Bay Area have a breast cancer rate
about 21 percent lower than that of their white counterparts, while
the rate for local Asian women is about half the rate for white
women.

A widely publicized report on the
rate of breast cancer in the Bay Area was issued in 1994 by the
Northern California Cancer Center, which gathers regional data for
the National Cancer Institute's Surveillance, Epidemiology and End
Results (SEER) program. Using data from an international study
comparing cancer rates in 20 countries around the world, the report
noted that white women in the Bay Area have a breast cancer rate 50
percent higher than women in most European countries and five times
higher than women in Japan. The report was covered extensively by
local media.

These regional differences in breast
cancer incidence are not a new phenomenon, said Robbins. He and
Kelsey based their analysis on incidence rates from the SEER
program for 1978 through 1982, plus risk factor data from a large
study conducted in roughly the same period, known as the Cancer and
Steroid Hormone (CASH) study. The CASH study gathered detailed
information on many known or suspected breast cancer risk factors
from women in eight SEER regions, including the Bay Area. Compared
with the other seven regions, the breast cancer rate in the Bay
Area was about 14 percent higher for white women and 10 percent
higher for black women.

"These differences have not changed
much from the early '80s into the '90s, so San Francisco does have
a persistently elevated breast cancer incidence," Robbins
said.

The use of data from 1980-82 is a
major strength of the new study, according to Robbins. Since that
time, the use of mammography for early detection of breast cancer
has increased dramatically, resulting in a large increase in breast
cancer incidence rates. The increase in mammography screening,
however, may not be uniform throughout the country or even among
different groups in the same region, and this makes it harder to
compare incidence rates.

"To a large extent, the incidence
rate nowadays is determined by how aggressively women are
encouraged to get screened, so we wanted to go back to a time when
screening would not confuse the issue," Robbins said.

In many respects, he found, Bay Area
women as a group ­ regardless of ethnic background ­
differed from women in the other seven SEER regions in ways that
would be expected to increase their risk of breast cancer. A host
of studies have linked a woman's risk of developing breast cancer
with certain reproductive factors, such as her age at the time of
her first menstruation, age at menopause, age at first pregnancy,
and number of children. Bay Area women, on average, differ from the
rest of the country in all of these characteristics.

Early menstruation, late menopause,
late age at first live birth, and low number of pregnancies all may
increase a woman's risk of breast cancer by affecting her lifetime
exposure to the hormone estrogen. For example, the risk of breast
cancer for a woman who bears her first child after age 30 is nearly
double that of a woman who bears her first child before age 20. The
average age at first full-term pregnancy in the Bay Area was 23.3
years, compared with 21.8 years in the other SEER regions. (These
figures may seem low, but they do not represent current
childbearing; rather, they represent the first pregnancies of women
20 to 55 years old who were surveyed in the early
1980s.)

Bay Area women, as a group, also
consume more alcohol than women in other parts of the country,
Robbins noted. The role of alcohol in breast cancer is
controversial, however, so he recalculated the analysis without
considering alcohol as a factor and found that this recalculation
did not substantially alter the outcome. For both black and white
women, adjusting for the prevalence of known risk factors
statistically eliminated the increased risk of breast cancer
associated with the Bay Area.

This study did not directly address
the issue of ethnic differences in breast cancer incidence.
Possible explanations for such ethnic variations, Robbins said,
include not only differences in reproductive factors, but also
genetic differences, dietary factors and differences in the use of
screening mammography.

The new findings should be
reassuring to women in the Bay Area, Robbins said. "The data," he
concluded, "are not consistent with an environmental cause for the
higher rate of breast cancer in the Bay Area, which is what many
women had feared."

Unfortunately, aside from alcohol
consumption, most of the risk factors that appear to account for
the Bay Area's higher breast cancer rate are difficult or
impossible for women to modify. In that respect, it is hard to take
much comfort from the study, Robbins acknowledged.

"However," he said, "the take-home
message is that there's no evidence the higher risk of breast
cancer in Bay Area women is due to the external environment. Women
don't need to fear that their risk will increase just because they
live here. I think that's important for women to know."

While this study may explain why
breast cancer is more common in the Bay Area than in other parts of
the nation, the higher rate of breast cancer in the United States
as a whole compared with other countries remains an issue of
intense interest to epidemiologists. Differences in diet, culture,
environmental factors, genetics and health care practices are among
the many factors that may be responsible, Robbins said.

He also noted that since breast
cancer rates throughout the United States are so much higher than
in other countries, any environmental causes of breast cancer in
this country would need to have a nationwide distribution in order
to explain the international differences. "To be consistent with
the data," Robbins said, "any serious environmental causes would
have to be affecting women in all regions of the United States, not
just women in the Bay Area." SR